E-315

Conjugation of the ubiquitin-like modifier SUMO (Sentrin) requires the activities of the heterodimeric E1 (SAE1/SAE2) and the UBE2I (Ubc9) E2 enzyme. The dimeric activating enzyme utilizes ATP to adenylate the C-terminal glycine residue of all SUMO proteins, forming a high- energy thiolester bond with the cysteine residue of SAE2 and the concomitant release of AMP and PPi. The second step is the trans-esterification reaction whereby SUMO is transferred to Cys93 of UBE2I.